This invention and embodiments thereof is directed to a medical field, and more particularly the field of radiological imaging devices. These devices comprise means for emitting radiation, such as an X ray source, and means for acquiring an image, such as an image detector, mounted on the end of a C shaped arm, and the object of which the image is to be taken is placed between the two sides of the arm. The images acquired by the detector as the arm rotates around the object, and thus several acquisitions correspond to different points of view of the object. Means for processing allow a three-dimensional model of the object to be reconstructed from the acquired two-dimensional images. This reconstruction supposes that the different positions of the device are known precisely as well as its geometric characteristics.
The three-dimensional model obtained can be used by a medical practitioner, such as a surgeon, before an operation in order to become familiar with the part of the anatomy for which the operation is intended. The three-dimensional model can also be used during the operation. For this, the medical practitioner disposes of equipment that allows the two-dimensional views of the part of the anatomy to be displayed in real time, these views being calculated from the three dimensional model.
The reconstruction of the three-dimensional model requires the imaging device to be “geometrically calibrated” beforehand. This calibration allows the three dimensional space to be linked to the two dimensional information provided by the various two dimensional projections. If the calibration is imprecise, then the quality of the three dimensional model reconstructed will reflect these imperfections.
A known calibration technique comprises placing markers inside an X ray field, positioned on a ghost image that act as markers in space, and carrying out a series of acquisitions. As the position of these markers in the three dimensional space is known, then the geometry of the acquisition can be deduced for each projection by inversion of a system of equations derived from the position of the markers on the projected images. A technique of this type is described, for example, in U.S. Pat. No. 5,442,674.
In general, when an image is taken of a part of the anatomy, the device is commanded to perform a series of acquisitions in the same geometrical conditions as the series of calibration acquisitions, so that the geometry of each acquisition is known precisely. This is possible due to the fact that the movements of the C shaped arm can be repeated.
If, however, the acquisitions of the part of the anatomy are not carried out in the same geometrical conditions as the calibration acquisitions (number of views, angular positions, speed of rotation, initial and final positions), then a new calibration has to be performed, which increases the number of views taken for the calibration.
Furthermore, the number of acquisitions required to obtain a good quality three-dimensional model depends on the type of anatomical structure to be imaged. Typically, the three dimensional reconstruction of the bone structures requires approximately 120 views whereas the three dimensional reconstruction of a blood vessel requires approximately 40 views.
The result is that as many calibration acquisitions as possible must always be taken so that the calibration is valid for imaging any type of structure.